chapter 6 panko and panko business data networks and security, 10 th edition, global edition...

Wireless LANs IChapter 6

Panko and PankoBusiness Data Networks and Security, 10th Edition, Global EditionCopyright © 2015 Pearson Education, Ltd.

Copyright © 2015 Pearson Education, Ltd.

Introduction

Radio Signal Propagation

Radio Bands and Bandwidth

Spread Spectrum Transmission

802.11 WLAN Operation

802.11 Transmission Standards

Wireless Mesh Networking


Ethernet 802.3 LANs

◦ Require standards at Layer 1 (physical) and Layer 2 (data link)

◦ Therefore, use OSI standards

◦ The 802.3 Working Group of the IEEE 802 Committee creates standards

6.1 802.11/Wi-Fi WLAN Technology


Wireless LANs

◦ Operate at Layers 1 and 2

◦ OSI dominates at Layers 1 and 2

◦ Therefore, they are OSI standards



802.11 Wireless LAN Technology

◦ The dominant WLAN technology today

◦ Standardized by the 802.11 Working Group of the IEEE 802 Committee



Wi-Fi Alliance◦ Industry association of 802.11 equipment

manufacturers

802.11 standards have many options

Wi-Fi Alliance selects subsets of standards as profiles

Does interoperability testing on these profiles

Only products that pass can display the Wi-Fi logo



Wi-Fi Alliance

◦ However, sometimes develops new standards

Two have been security nightmares

WPA

WPS



6.2 Hybrid Switched/Wireless 802.11 Network

The wireless access point connects thewireless client to the wired Ethernet LAN.


6.2 Hybrid Switched/Wireless 802.11 Network

The LAN connection is needed to give clients access to

servers and Internet access routers on the wired LAN.


Introduction








6.3 Electromagnetic Wave Optical fiber

transmission is measured in

terms of wavelength.

Typical data LAN frequencies are 500 MHz to 10 GHz


6.4 Omnidirectional and Dish Antennas


6.4 Omnidirectional and Dish Antennas

Questions:What type of antenna do mobile phones use?

Why?


6.5: Wireless Propagation Problems

2.Electromagnetic

Interference (EMI) is

unwanted power at the same

frequency from other devices.

1.Wireless

transmission has many

propagation problems.



Shadow zones are places the signal cannot penetrate because of obstacles in its path.

Shadow zones, also called dead spots, grow worse as frequency increases.


6.5: Wireless Propagation ProblemsThe signal strength

spreads out as the surface of a sphere.

This means that its strength falls as

(1/r2), where r is the radius.

If you double the distance, you only get ¼ the signal

strength

Radio signals spread out in a sphere.◦ S = signal power, r = range (distance) or radius

◦ If the signal power at 10 meters is 9 milliwatts (mW), how strong is it at 30 meters?


6.6: Inverse Square Law Attenuation

S2 = S1 * (r1/r2)2

S2 = 9 mW * (10/30)2

S2 = 9 mW * (1/3)2

S2 = 9 mW * (1/9)

S2 = 1 mW

Your turn.

◦ If the signal strength at 5 meters is 48 mW, how strong is it at 20 meters?


6.6: Inverse Square Law Attenuation



Signal is absorbed by the air and water.

Note that there are two types of attenuation: Inverse square law and absorptive attenuation.

Note that this is different than shadow zones.



Direct and reflected signals may interfere.

Most serious propagation problem at WLAN frequencies.

6.7: Multipath Interference


If the two waves are out of phase, they will

negate each other, giving no signal.



RecapTwo forms of attenuation

Two grow worse at higher frequencies


Introduction








6.8 Frequency Spectrum, Service Band, Channels

0 Hz


6.8 Frequency Spectrum, Service Band, Channels

台灣的無線電波波譜分配圖表

© 2011 Pearson Education, Inc. Publishing as Prentice Hall 27


6.9 Signal Bandwidth

Channel Bandwidth

◦ Channel bandwidth is the highest frequency in a channel minus the lowest frequency.

◦ An 88.0 MHz to 88.2 MHz channel has a bandwidth of 0.2 MHz (200 kHz).

◦ Higher-speed signals need wider channel bandwidths.


6.10: Channel Bandwidth and Transmission Speed


Transmission Speed and Channel Bandwidth

◦ There is a direct relationship between required transmission speed and required channel bandwidth

◦ Doubling bandwidth doubles the possible transmission speed

◦ Multiplying bandwidth by N makes possible N times the transmission speed

6.10 Channel Bandwidth and Transmission Speed


Broadband Channels

◦ Broadband means wide radio channel bandwidth and therefore high speed

◦ Popularly, fast systems are called “broadband” even if they are not radio systems

6.10 Channel Bandwidth and Transmission Speed


The 2.4 GHz Service Band

◦ 2.4 GHz to 2.485 GHz

◦ Propagation characteristics are good

◦ For 20 MHz 802.11 channels, only three nonoverlapping channels are possible

◦ Channels 1, 6, and 11

6.11 2.4 GHz and 5 GHz Service Band


http://en.wikipedia.org/wiki/IEEE_802.11

http://en.wikipedia.org/wiki/IEEE_802.11


The 2.4 GHz Service Band

◦ Co-channel interference between nearby access points transmitting in the same channel degrades performance

◦ Except in very small networks, difficult or impossible to put nearby access points on different channels (Figure 6-12)



The 5 GHz Service Band

◦ More bandwidth, so between 11 and 24 non-overlapping 20 MHz channels

◦ Makes it easy to have nearby access points operate on non-overlapping channels

◦ Increasing channel bandwidth in newer standards reduces the number of possible channels



What is the main advantage of 2.4 GHz operation?

What is the main advantage of 5 GHz operation?



6.12 Co-Channel Interference, 2.4 GHz Band The 2.4 GHz Unlicensed Band

◦ Difficult or impossible to put nearby access points on different channels

Figure 6-12


Licensed Radio Bands◦If two nearby radio hosts transmit in the same

channel, their signals will interfere

◦Most radio bands are licensed bands, in which hosts need a license to transmit: FM radio, etc.

◦The government limits licenses to control interference

◦In cellular telephone bands, only the central antennas are licensed, not the mobile phones

6.13 Licensed & Unlicensed Service Bands


Unlicensed Radio Bands

◦ A few service bands are set aside as unlicensed bands

◦ Hosts in these service bands do not need to be licensed to be turned on or moved

◦ Anybody can operate anywhere




◦ 802.11 Wi-Fi operates in unlicensed radio bands

◦ This allows access points and hosts to be moved freely




◦ However, there is no legal recourse against interference from other nearby users

◦ Your only recourse is to negotiate

◦ At the same time, you may not cause unreasonable interference by transmitting at illegally high power



Introduction








◦ You are required by law to use spread spectrum transmission in the 2.4 and 5 GHz bands.

◦ Spread spectrum transmission reduces propagation problems. Especially multipath interference

◦ Spread spectrum transmission is NOT used for security in WLANs.




6.14: Normal vs Spread Spectrum Transmission

Why do you not want to make radio channel bandwidth greater than the signal

requires?


6.14: Normal vs Spread Spectrum Transmission

Wasteful of bandwidth but necessary

to avoid propagation problems.


6.15: Orthogonal Frequency Division Multiplexing


Introduction



Spread Spectrum Transmission802.11 WLAN Operation802.11 Transmission Standards



6.16 Typical 802.11 Wi-Fi Operation

Sender puts a packet for the destination host into an 802.11 frame, then sends the

frame wirelessly to the access point.



The 802.11 frame has the wrong frame format to travel over an 802.3 Ethernet network. The switches and destination host would not know what to do with

it.

The access point removes the packet from the 802.11 frame and discards the frame.



The access point encapsulates the packet in an 802.3 frame and sends this frame on to the

destination host via Ethernet switches.



The server removes the packet from the 802.3 frame.

The Wired Ethernet Network is called the Distribution System

Does the IP packet travel all the way to the destination host?

Does the 802.11 frame travel all the way to the destination host? Why or why not?



An access point and its wireless hosts

Service Set ID (SSID) is the name of the network (abc)

6-17: Basic Service Set (BSS)


Collection of access points that all have the same SSID

6-17: Extended Service Set (ESS)



6.18: Transmitting in a Single Channel

Carrier Sense Multiple Access with Collision Avoidance and Acknowledgement◦ Mandatory for 802.11 Wi-Fi Operation

Carrier Sensing with Multiple Access◦ Sender listens for traffic (senses the carrier)

◦ If another device is transmitting, it waits

◦ This controls access by multiple devices that must not transmit simultaneously


6.19: CSMA/CA+ACK Box

Collision Avoidance

◦ When the current sender stops, two or more waiting devices may immediately want to transmit

◦ This will cause a collision

◦ Instead, the devices must wait a randomized amount of time before sending

◦ This usually avoids collision, but it is inefficientCopyright © 2015 Pearson Education, Ltd.


ACK (Acknowledgement) and Reliability

◦ Receiver immediately sends back an acknowledgement

◦ CA random delay for other devices guarantees there will be enough time for an immediate ACK

◦ If sender does not receive the acknowledgement, it retransmits using CSMA



ACK (Acknowledgement) and Reliability

◦ CSMA/CA plus ACK is a reliable protocol

◦ Reliable protocols are rare in wired networks

◦ However, radio transmission is unreliable enough to warrant it



Inefficiency

◦ There is a lot of waiting with CSMA/CA+ACK

◦ This makes it inefficient



Hidden Node Problem(or Hidden Terminal Problem)


http://en.wikipedia.org/wiki/Hidden_node_problem

http://en.wikipedia.org/wiki/Hidden_node_problem


6.20: RTS-CTSBox


6.20: RTS-CTS

0

Box

CSMA/CA is Mandatory◦ It is the default MAC method.

◦ It is more efficient than RTS/CTS.

RTS/CTS◦ Is usually optional.

◦ Is good if two or more client stations cannot hear each other.

◦ Stop them from transmitting at the same time.


ComparisonBox


Introduction





802.11 Transmission StandardsWireless Mesh Networking


Standard Rated Speed

802.11g 54 Mbps

802.11a 54 Mbps

802.11n 100 to 600 Mbps150 to 300 Mbps common

802.11ac 433 Mbps to 6.93 Gbps433 to 1.3 Gbps common

6.21 Major 802.11 Wi-Fi Standards


Standard Status

802.11g Obsolescent but still used

802.11a Obsolescent but still used

802.11n Dominates the installed base today

802.11ac Dominates sales today



Standard Unlicensed Bands

802.11g 2.4 GHz

802.11a 5 GHz

802.11n 2.4 GHz and 5 GHz

802.11ac 5 GHz



Standard Channel Bandwidth

802.11g 20 MHz

802.11a 20 MHz

802.11n 40 MHz but will drop back to 20 MHz if there is interference from a 802.1g or 802.1a channel

802.11ac 80 MHz and 160 MHz



Standard Channels in 5 GHz Band in the USA

802.11g NA (2.4 GHz only)

802.11a 20-25

802.11n 20-25 at 20 MHz8-12 at 40 MHz

802.11ac 4-6 at 80 MHz1-2 at 160 MHz


Wider channels

means fewer channels

6.22: MIMO


Access point transmits two signals in the same channel—one from Antenna A and

one from Antenna B.These are called spatial streams.

6.22: MIMO


The two signals arrive at different times at the two receiving antennas. Time

differences allow them to be separated and understood.

MIMO Two Benefits

◦ MIMO brings higher speeds because it can send more information in a channel.

◦ MIMO also brings longer propagation distances for technical reasons we will not discuss.


6.22: MIMO (Multiple Input/Multiple Output)


6.23: Beam Forming

Beamforming allows an access point to focus its transmissions and

reception

6.23: Beam Forming


Multiuser MIMO (MU-MIMO) allows two wireless hosts to transmit at the same time.

Defined in 802.11n, but a single method was not defined

Defined in 802.11ac, and there is a single standard, so adoption is more likely

However, initial pre-standard 802.11ac products typically do not offer it.

Multiuser MIMO



Standard MIMO? Maximum Spatial Streams

MU-MIMO?

802.11g No NA No

802.11a No NA No

802.11n Yes 4 No

802.11ac Yes 8 Yes



Rated Speed versus Throughput

◦ Rated speed is the speed the standard specifies

◦ Throughput is the speed that is actually delivered

◦ Total throughput is substantially lower than rated speed—sometimes 50% lower

6.24 Speed, Distance, and Throughput


Aggregate versus Individual Throughput

◦ Access point throughput is aggregate throughput for all devices transmitting and receiving

◦ Its capacity is shared by all stations currently sending or receiving

◦ Individual throughput can be much lower than aggregate throughput



Throughput versus Distance

◦ As distance from the access point increases, signals get weaker

◦ Wireless hosts must use slower but more reliable bit encoding methods—send fewer bits per clock cycle

◦ This reduces individual throughput for these devices



Devices Built for Newer Standards Still Implement Older Standards

◦ This is called backward compatibility

◦ This Allows Older and Newer Devices to Communicate

6.25 Backward Compatibility


Example of Backward Compatibility

◦ Newer product: 802.11ac in the 5 GHz band

◦ Older product: 802.11n in the 2.4 GHz band

◦ The newer product will drop back to the standard the older product can use

◦ They will communicate using the 802.11n standard in the 2.4 GHz band

◦ They will only get 802.11n speeds

6.25 Backward Compatibility


Introduction





802.11 Transmission StandardsWireless Mesh Networking


6.26: Wireless Mesh Network

There is no Ethernet network(distribution system)

Frames are forwarded by access pointsand wireless hosts


6.26: Wireless Mesh Network

Mesh networks are governed by 802.11s

It is not a mature standard

chapter 6 panko and panko business data networks and security, 10 th edition, global edition...

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